1. Field of the Invention
The present invention relates to a charge transfer device, and more specifically to an output circuit for a charge transfer device.
2. Description of Related Art Charge transfer devices are widely used in various fields, for example as a photo-electric conversion device incorporated in facsimiles and television cameras.
One typical example of conventional charge transfer devices is disclosed in U.S. Pat. No. 4,646,119, a content of which is incorporated therein by reference thereto. In brief, in typical conventional charge transfer devices, a signal charge is transferred through moving potential wells created by a multi-phase clocks, so as to fed into a floating diffusion region between an output gate and a reset gate. The signal charge temporarily accumulated in the floating diffusion is read out to an associated read-out transistor by previously resetting the reset gate so as to bring the potential of the floating diffusion region to a reset voltage, and then by closing the reset gate so as to allow the signal charge to be accumulated in the floating diffusion region from a signal charge transfer section, so that the potential of the floating diffusion region changes from the reset voltage by the amount corresponding to the signal charge. This amount of change in the potential of the floating diffusion region is detected by the associated read-out transistor and outputted as a voltage change signal from the associated read-out transistor.
Here, assuming that the signal charge is expressed by "Q" and the total capacitance of the floating diffusion region is shown by "C", the amount of change .DELTA.V in the potential of the floating diffusion region can be defined by .DELTA.V=Q/C. Namely, the detection sensitivity can be improved by reducing the total capacitance of the floating diffusion region. Here, the total capacitance of the floating diffusion region includes a capacitance between the floating diffusion region and a substrate, a capacitance between the floating diffusion region and the output gate, a capacitance between the floating diffusion region and the reset gate, and a capacitance between a gate electrode and a drain of the read-out transistor connected to the floating diffusion region.
In the prior art, therefore, in order to realize a high detection sensitivity of the charge transfer device, the plan pattern of the output circuit for the charge transfer device is reduced in size so as to decrease the total capacitance of the floating diffusion region. However, this approach has a limit as a matter of course.